The ruling Wednesday by U.S. District Judge William Alsup of San Francisco allows the National Union of Healthcare Workers to seek court orders declaring Kaiser's conduct unlawful and prohibiting it in future elections.

The SEIU is the nation's largest private-sector union and has about 150,000 members in California. Dissident members of an SEIU local formed the NUHW in 2009 and have been trying to dislodge the larger union as representative of 45,000 Kaiser employees in the state.

A 2010 SEIU victory among Kaiser workers was overturned by the National Labor Relations Board, which found evidence of collusion between the company and the union. A second election was held in April and the SEIU again defeated the NUHW, winning 58 percent of the vote.

The smaller union's lawsuit centered on Kaiser's practice, under its contract with the SEIU, of paying certain benefits to union members while they take a one-year leave of absence to work for the union.

For several years, Kaiser made those payments to only a handful of employees who were performing contract-related tasks, the NUHW said. But before both the 2010 and 2013 elections, the suit said, the company released and paid hundreds of SEIU members to take time off and campaign for their union.

Kaiser denied wrongdoing and said the National Labor Relations Board had found that any payments it made had no impact on the 2010 election. The company also argued that the case was moot because the NLRB has certified this year's election.

But Alsup said the dispute was likely to recur in the next election, which could take place in 2015. And while courts have allowed employers to pay benefits to union representatives, he said, no court has upheld such payments for campaigning by an incumbent union against a rival.

"One evil Congress wished to resist (in labor law) was a sweetheart cozy arrangement between the incumbent union boss and the company," Alsup said. If the NUHW's allegations are accurate, he said, Kaiser effectively made "cash campaign contributions" to the SEIU, unauthorized by its contract.

NUHW President Sal Rosselli said the ruling was evidence that the SEIU is an "illegal company union."

Steve Trossman, a spokesman for SEIU's United Healthcare Workers, shot back that the lawsuit was "a PR stunt by an organization that twice has been soundly rejected by Kaiser workers." Kaiser said Thursday that the smaller union's allegations are baseless and that the suit will ultimately fail.